1. Field of the Invention
This invention relates to water reducible polymeric coating compositions which can be applied to a variety of substrates, and in particular, to a coating composition that is useful in automotive finishes.
2. Description of the Prior Art
Polyurethanes constitute a broad class of polymeric materials having a wide range of physical characteristics. The polymers are usually produced through the interaction of a polyfunctional isocyanate with a polyfunctional chemical compound having an active hydrogen in its structure.
In U.S. Pat. Nos. 3,865,898, 3,257,476 and 3,291,859 there are disclosed block copolymers and processes for forming them. In U.S. Pat. Nos. 3,257,476 and 3,291,859 the block copolymers are of an A-B-A structure. These patents teach a synthesis route to the preparation of block copolymers wherein one block is a vinyl copolymer and the other block can generally be referred to as a polyurethane. In the general scheme of synthesis as taught by these patents, a prepolymer is formed by reacting an aromatic diisocyanate with a polymeric material having functional groups with active hydrogen to form an "isocyanate-capped" prepolymer. The prepolymer is then usually reacted with tert-butyl hydroperoxide, a cumene hydroperoxide, or a dual-functional free radical initiator to form a peroxycarbamate which has reactive sites capable of initiating the polymerization of ethylenically-unsaturated monomers to form a block copolymer.
Water dispersible polyurethanes are also known in the art. For example, U.S. Pat. No. 2,968,575 to Mallonee discloses emulsifying NCO-containing prepolymers in a solution of diamine and water with the aid of detergents. A chain lengthening reaction takes place as water and diamine diffuse into the droplets of emulsion and react with the isocyanate.
There have also been suggestions in the prior art to prepare fully reacted polyurethanes in organic solvent with internally contained salt groups which permit the polyurethane to be dispersed in water. For example, U.S. Pat. No. 3,479,310 to Dieterich et al, discloses dispersing a fully chain extended, NCO-free polyurethane having internally contained ionic salt groups in water without the aid of detergent.
I have now found that a water reducible fully reacted polyurethane of the type described in the aforementioned patents can be produced which in turn can be used for the in situ polymerization with one or more ethylenically unsaturated monomers to form a water reducible acrylic-urethane polymer. The acrylic-urethane polymer can be coated from solution to form a tough, durable coating that can be air dried, heat cured or cross-linked, and has particular utility, when combined with pigments and metallics, in automotive finishes.